Modern cellular base stations are separated into two units, a baseband unit and a remote radio head. The baseband unit, or baseband system, is located on the ground, often proximate to a tower. The baseband unit modulates and demodulates digital data. Thus, for example, the baseband unit includes with a modulator and demodulator.
The remote radio head coupled to, and mounted proximate to, one or more antennas on a tower. The remote radio head down converts and up converts the digital data to radio frequencies and amplifies received and transmitted radio frequency signals. This respectively enhances reception sensitivity and broadcast power of the cellular base stations. Thus, for example, the remote radio head includes an upconverter, a down converter, a low noise amplifier, and a power amplifier.
However, the use of a remote radio head requires that power be supplied to the remote radio head, on the tower, through a power cable. However, as disclosed in U.S. Pat. No. 9,448,576, because the length of the cable can be hundreds of feet and the current drawn by the remote radio head can be, e.g., about 20 Amperes at a voltage level of about fifty volts, the power loss due to the power cable can be significant. U.S. Pat. No. 9,448,576 is incorporated by reference herein as if set forth in its entirety.
Power loss attributable to the power cable is problematic if the cellular base station needs to operate on backup battery power in the event of a power blackout. The power dissipated by the power cable can be sufficiently large to undesirably reduce the operating time of the cellular base station when powered by a battery backup system. An equally undesirable alternative, due to increased cost, would be to compensate for the loss by increasing the number of batteries in the battery backup system.
To reduce the power loss, the voltage provided to the power cable can be increased to proportionally reduce the current that must be provided to power the remote radio head. The reduction in current reduces power dissipation by the square of the current reduction, or voltage increase.
However, the power consumption of the remote radio head varies during its normal operation. As a result, the current drawn and hence the voltage provided to the remote radio head vary over time. To maintain a constant voltage at the input of the remote radio head, the resistance of the power cable must be known. Although the resistance can be measured when the cellular base station is constructed, the resistance can change over time due to environmental effects. Thus, there is a need to periodically measure the resistance of the power cable.
U.S. Pat. No. 9,448,576 describes adding circuitry to a power supply to characterize the resistance of the power cable. However, this increases the cost of the cellular base station. Therefore, there is a need for a less costly means of determining the resistance of the power cable.